The invention pertains to footwear, and in particular to athletic footwear used for running. More specifically, the present invention pertains to athletic shoe constructions designed to attenuate force applications and shock and to enhance stability upon rearfoot strike during running.
The modern athletic shoe is a highly refined combination of elements which cooperatively interact in an effort to minimize weight while maximizing comfort, cushioning, stability and durability. However, these goals are potentially in conflict with each other in that efforts to achieve one of the objectives can have a deleterious effect on one or more of the others. As a result, the shoe industry has continued in its efforts to optimize these competing concerns. These efforts have in large part been directed at optimizing the competing qualities of cushioning and stability.
In modern athletic shoes, the sole ordinarily has a multi-layer construction comprised of an outsole, a midsole and an insole. The outsole is normally formed of a durable material such as rubber to resist wearing of the sole during use. In many cases, the outsole includes lugs, cleats or other elements to enhance traction. The midsole ordinarily forms the middle layer of the sole and is typically composed of a soft foam material to cushion the impact forces experienced by the foot during athletic activities. An insole layer is usually a thin padded member provided over the top of the midsole to enhance shoe comfort.
Up until the 1970's, athletic shoes were by and large considered deficient in providing cushioning for the wearer's foot. Consequently, numerous foot related injuries were sustained by those engaging in athletic activities. To overcome these shortcomings, over the ensuing years manufacturers focused their attention upon enhancing the cushioning provided by athletic shoes. To this end, midsoles have over time increased in thickness. These endeavors have further led to the incorporation of special cushioning elements within the midsoles intended to provide enhanced cushioning effects. In particular, the use of resilient inflated bladder midsole inserts, e.g., in accordance with the teachings of U.S. Pat. Nos. 4,183,156, 4,219,945, 4,340,626 to Rudy, and U.S. Pat. No. 4,813,302 to Parker et al., represents a marked improvement in midsole design and has met with great commercial success. (These patents are hereby incorporated by reference herein.) The industry's focus on improving cushioning effect has greatly advanced the state of the art in athletic shoe design. In some cases, however, the benefits realized in cushioning have been offset by a degradation of shoe stability.
To appreciate the potentially harmful effects of shoe instability, it is important to have a basic understanding of the dynamics of running and the anatomy of the foot. While the general population includes a wide variety of running styles, about 80% of the population runs in a heel-to-toe manner. In this prevalent running style, the foot does not normally engage the ground in a simple back to front linear motion.
When most persons run, their feet generally engage the ground under the approximate midline of their body, rather than to the sides as in walking. As a result, the foot is tilted upon ground contact such that initial engagement with the ground (commonly referred to as rearfoot strike or heel strike) usually occurs on the lateral rear corner of the heel. (See FIG. 1.) At heel strike, the foot is ordinarily dorsi flexed and slightly inverted. Typically, the ankle angle .alpha. is within approximately between 7.degree. plantarflexion and 12.degree. dorsiflexion, and the angle of inversion .beta. is approximately 6.degree.. Furthermore, at heel strike the foot is typically abducted outwardly from the straight forward direction (A) at an angle .gamma. from 10.degree. to 14.degree.. In this respect, see also U.S. Pat. No. 4,439,936 to Clark et at., which is hereby incorporated by reference herein. As the ground support phase progresses, the foot is lowered to the ground in a rotative motion such that the sole comes to be placed squarely against the ground. Inward rotation of the foot is known as eversion, and in particular, inward rotation of the calcaneus associated with articulation of the sub-talar joint is known as rearfoot pronation. While eversion is itself a natural action, excessive rearfoot pronation, or an excessive rate of pronation is sometimes associated with injuries among runners and other athletes.
Referring to FIGS. 2 and 3, it is seen that the foot is interconnected to the leg via the tarsus (the posterior group of foot bones). More specifically, the tibia 1 and fibula 3 (i.e., the leg bones) are movably attached to the talus 5 to form the ankle joint. In general, the leg bones 1, 3 form a mortise into which a portion of talus 5 is received to form a hinge-type joint which allows both dorsi flexion (upward movement) and plantar flexion (downward movement) of the foot. Talus 5 overlies and is movably interconnected to the calcaneus 7 (i.e., the heel bone) to form the sub-talar joint. The sub-talar joint enables the foot to move in a generally rotative, side to side motion. Rearfoot pronation and supination of the foot is generally defined by movement about this joint. Along with talus 5 and calcaneus 7, the tarsus further includes navicular 9, cuboid 11 and the outer, middle and inner cuneiforms 13, 15 and 17. The cuboid and cuneiforms facilitate interconnection of the tarsus to the metatarsals (the middle group of foot bones). Generally, the rearfoot area is considered to extend to the junction 19 between the calcaneus 7 and cuboid 11.
As mentioned, an industry trend has been toward thickening the midsoles of athletic shoes to enhance the cushioning effect of the sole. An added thickness of foam, however, can cause the sole to have increased stiffness in bending. Under these conditions, the lateral rear corner of the sole can tend to operate as a fulcrum upon heel strike and create an extended lever arm and greater moment, which can cause the foot to rotate medially and pronate with greater velocity than is desirable. This can lead to over-pronation of the foot and possible injury. Further, this condition can present a potentially unstable condition for the foot and results in the transmission of higher than desired levels of impact stress due to the relatively small surface area of contact and the relative stiffness of a conventional sole having a higher density foam sidewall, and therefore greater stiffness in the area of heel strike.
The footwear industry has wrestled with the aforementioned bio-mechanical phenomena associated with rearfoot strike for years, and various strategies have been directed towards reducing rearfoot impact shock, increasing stability and/or discouraging over-pronation.
It is known to use deep grooves, channels or slits in order to increase sole flexibility in the heel area. Two early teachings involve segmentation of a rigid sole of a street shoe, in order to reduce heel shock and to promote a more natural walking action. See Stein U.S. Pat. No. 2,629,189 and German Patent No. 680,698 to Thomsen et al. (1939). More recent teachings involving athletic shoes are disclosed in Hunt U.S. Pat. No. 4,309,832; Riggs U.S. Pat. No. 4,638,577; and Ellis PCT Applications Nos. WO 91/05491, WO 92/07483, WO 91/11924 and WO 91/19429.
Another approach taken in the prior art for minimizing the shock and over-pronation associated with heel strike involves the use of a relatively compliant midsole material in a lateral heel area and a stiffer material on a medial side. See, e.g., Cavanagh U.S. Pat. No. 4,506,462 and Bates U.S. Pat. No. 4,364,189.
The above-described segmented soles of the prior art do not adequately address the aforementioned heel strike dynamics of most runners. Typically, the application to shoe soles of grooves, slits, and materials exhibiting differential cushioning characteristics have involved excessively large heel and midfoot regions, whereby less than ideal medial and lateral stability results. In other words, the prior art has failed to properly delimit a rearfoot strike zone wherein heel strike occurs with the vast majority of runners. Through the misplacement or over placement of flex grooves or the like, medial and lateral instability in the heel and mid-foot regions can result. Similarly, the extension of a softer sole material beyond the critical heel strike area about medial and lateral sides of the heel can adversely affect footwear stability.
It is known to incorporate into the sole of a running shoe cushioning elements including resilient inflated bladders, such as taught in the aforementioned Rudy U.S. Pat. Nos. 4,183,156, 4,340,626 and 4,219,945, and U.S. Pat. No. 4,817,304 to Parker et al. Soles incorporating gas filled bladder elements in accordance with these patents represent a great advance in athletic footwear cushioning technology. They provide a significant improvement in protection from impact stress as compared with soles formed of conventional plastic foam, by exhibiting a more linear spring characteristic throughout their range of compression and thereby transmitting lower levels of shock to a wearer during use. They also have the advantage of significantly reduced weight. Additionally, soles in accordance with the aforementioned patents have proven to be highly durable and long lasting. Conventional foam soles .can break down and take on compression set after a relatively short period of usage. The inclusion of a resilient fluid bladder in the sole greatly reduces compression set due to the reduced reliance on degradable foam plastic to provide a cushioning effect.
The aforementioned Ellis PCT application No. WO 91/11924 discloses the adaptation of a conventional gas filled bladder cushioning device to a sole including spaced longitudinal deformation sipes (slits or grooves). In this embodiment, the gas-filled devices are unconnected tube-shaped chambers located in parallel and between the deformation sipes. The disclosed arrangement would provide substantially uniform flexibility and cushioning across the entire heel area, including the medial side, thus possibly resulting in a degradation of medial stability and a tendency towards over-pronation. Additionally, the longitudinal orientation of the sipes would not provide optimal articulation of the heel area to attenuate shock on rearfoot strike.
A prior art NIKE.RTM. walking shoe (the AIR PROGRESS.RTM.) has a single deep flex groove running substantially transversely across the sole in the heel area. A segmented gas filled bladder has chambers in fluid communication positioned on either side of the groove, and an area of enhanced flexibility aligned with the flex groove. This shoe advantageously provides some of the improved cushioning characteristics that a gas-filled bladder can afford, while allowing relatively unimpeded articulation about the hinge line. While this shoe works well for walking, which typically involves a heel strike centered about the longitudinal axis of the sole, the strike zone is not properly delimited to account for rearfoot strike during running. Furthermore, the sole does not provide differential cushioning in different zones to attenuate force applications and shock while at the same time enhancing stability.
It is known to incorporate into an athletic shoe relatively rigid motion control elements for controlling pronation and stabilizing the heel. For example, U.S. Pat. No. 5,046,267 to Kilgore et at. (incorporated by reference herein) discloses a plastic motion control device (FOOTBRIDGE.RTM.) incorporated into a midsole and extending across the footbed in order to gradually increase the resistance to compression of the midsole from the lateral side to a maximum along the medial side, and thereby control rearfoot pronation.
So-called heel counters are commonly incorporated into athletic and other shoes for properly positioning and providing stability to the heel and arch of the foot. Heel counters are generally formed of relatively rigid material (as compared to the primary upper and midsole materials) and extend upwardly from the sole co-extensive with a portion of the upper, in the heel area on both lateral and medial sides thereof. Typically, a heel counter will surround or cup the heel as a single rigid piece. An integrally formed rearfoot motion control device (FOOTBRIDGE.RTM.) and heel support (heel counter) is disclosed in the present Assignee's copending application Ser No. 07/659,175 (incorporated by reference herein).
The Nike.RTM. AIR HUARACHE.RTM. has a heel counter which is split into upstanding lateral and medial panel portions affixed to the upper in the region of the heel. This shoe sole has a conventional sole including a gas filled bladder, without means for providing differential cushioning and/or independent articulation between a rearfoot strike zone and a remaining heel area.
U.S. Pat. Nos. 4,445,283 and 4,297,797 to Meyers disclose the use of a relatively firm fluid tight chamber in a medial heel area of a sole and a relatively compressible chamber in a lateral heel area, so as to create greater weight bearing on the lateral side such that the medial side may form a supportive arch when the lateral side deforms. The Meyers bladder also includes a transversely extending groove or split in a midfoot region for providing flexibility. Meyers does not delimit an articulated rearfoot strike zone reflecting the dynamics and location of heel strike in most runners.
Coomer U.S. Pat. No. 4,305,212 discloses an arrangement of gas filled bladders having differential pressures in different parts of the heel area of the sole. Central lower pressure zones are surrounded by a high pressure zone extending about the rear part of the sole from a lateral to medial side, in order to capture or catch the heel in a neutral position. Due to the increased pressure in the area where heel strike will occur, less than ideal attenuation of force applications and shock on heel strike would result. Furthermore, the design does not delimit an articulated rearfoot strike zone reflecting the dynamics and location of heel strike in most runners.